Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination system, a reticle stage assembly that retains a reticle, a projection optical assembly and a wafer stage assembly that retains a semiconductor wafer. The illumination system includes an illumination source and an illumination optical assembly. The reticle stage assembly and the wafer stage assembly are supported above a ground with an apparatus frame.
Typically, the wafer stage assembly includes a wafer stage base, a wafer stage that retains the wafer, and a wafer stage mover assembly that precisely positions the wafer stage and the wafer below the projection optical assembly. Somewhat similarly, the reticle stage assembly includes a reticle stage base, a reticle stage that retains the reticle, and a reticle stage mover assembly that precisely positions the reticle stage and the reticle between the illumination optical assembly and the projection optical assembly. The size of the images and features within the images transferred onto the wafer from the reticle are extremely small. Accordingly, the precise positioning of the wafer and the reticle relative to the lens assembly is critical to the manufacture of high density, semiconductor wafers.
Depending upon the type of energy beam generated by the illumination source, the type of fluid surrounding the reticle and the wafer can influence the performance of the exposure apparatus. For example, some types of beams, e.g. electron beams and very short wavelengths of ultraviolet light, are absorbed by oxygen and other gases. Thus, the environment surrounding the reticle and wafer can influence the performance of the exposure apparatus and the quality of the integrated circuits formed on the wafer can be compromised. As a result thereof, the performance of the exposure apparatus and the quality of the integrated circuits formed on the wafer can be enhanced by controlling the environment around one or both stages.
One way to control the environment around a stage is to position a chamber around the stage. Subsequently, the desired environment can be created within the chamber around the stage. For example, for some processes, the chamber may be filled with an inert fluid. Alternately, electron beam processes function best when the controlled environment is a vacuum.
Historically, stage assemblies used in a vacuum environment have utilized mechanical type bearings to support the stage. Typical mechanical type bearings include ball bearings, roller bearings or sliding contact. However, limitations on the use of lubricants in a vacuum, rolling or sliding noise or vibration, particle generation, and friction also limit the accuracy and velocity of such stages.
One solution is to use an air bearing in the vacuum to support the stage. However, air bearings typically require substantial preload forces to have high stiffness, which is desirable for precision stages. Unfortunately, it is not possible to create a vacuum preload type air bearing if the stage is surrounded by a vacuum.
Alternately, a lower air bearings and an opposed upper air bearing can be used to support the stage in the vacuum environment. In this embodiment, the upper air bearing preloads the lower air bearing to create a relatively stiff bearing. However, this design typically requires an increase in stage mass and/or complexity and an increase in the number of air bearings required by the stage assembly. In addition, the opposed air bearings can deform the stage.
Moreover, existing reticle stage mover assemblies include one or more moving motors that generate stray magnetic fields. Unfortunately, the stray magnetic fields of significant magnitude can influence the electron beam. Thus, with current reticle stage assemblies, the motors can influence the electron beam.
Additionally, in an electron beam exposure apparatus, a relatively narrow vertical gap exists between the illumination optical assembly and the projection optical assembly for positioning the reticle. Unfortunately, for existing reticle stage assemblies, the combination of the reticle stage base and the reticle stage is relatively thick. Further, existing reticle and wafer stage assemblies can have significant perturbations from the drag from control cables and hoses attached to the moving stage. As a result thereof, the perturbations can cause an alignment error between the reticle and the wafer. This reduces the accuracy of positioning of the wafer relative to the reticle and degrades the accuracy of the exposure apparatus.
In light of the above, there is a need for a stage assembly having relatively high acceleration and velocity capabilities that precisely positions a device. Another need is to provide a stage assembly having a stage that is relatively lightweight and relatively thin and that has relatively high modal frequencies. Further, there is a need for a stage assembly for positioning a device in a controlled environment such as a vacuum. Moreover, there is a need for a stage assembly that minimizes the perturbations on the stage caused by cable and hose drag and has relatively small stray magnetic fields. Additionally, there is a need for a stage assembly that does not include moving magnets or iron which will cause perturbations in existing magnetic fields. Further, there is a need for a stage assembly that minimizes moving conductors which can create eddy currents in existing magnetic fields and thereby alter the existing magnetic fields. Moreover, there is a need to provide a high performance stage assembly for an exposure apparatus that utilizes an electron beam. Furthermore, there is a need for an exposure apparatus capable of manufacturing precision objects, such as high density, semiconductor wafers.